@Robbie if inschrijven.datumEvenement is from a Date/Time field than it is already a DateTime object and you don't need the date filter or date function. But even if it is not, you never need both! :D
– carlcsMay 15 '15 at 21:32

@carlcs I thought so too but I was seeing incorrect results without the date function. For example {{ globalSet.dateField.diff( now ).format('%a') }} resulted in (unknown) and {{ globalSet.dateField.diff( now ).days }} outputs -99999. And using the date function without the date filter also produced the same results. Do you know why that would be?
– Dylan TMay 15 '15 at 21:56

Hmm.. it could be possible that Craft DateTime objects don't do the diff method or do something wrong, and the date filter converts them to normal Twig DateTime objects?
– carlcsMay 15 '15 at 22:11

Ok, it is, as I guessed, a bug with the Craft DateTime object. Normal DateTime object you create using Twig's date function work just fine: date('2015-05-01').diff(date('2015-05-04')).days. What you actually do with combining date filter and function is to return the Craft DateTime as a string and then make that a normal Twig DateTime using the function. One thing to note to that "workaround" is that you should better return the DateTime including the time and the timezone with the filter, e.g. date(myDate|date('c')) or date(myDate|date('U')).
– carlcsMay 15 '15 at 23:18